1. Technical Field
The present invention relates to very high solid content aerosols, and, more particularly, to very high solid content aerosol adhesives and aerosol adhesive application devices.
2. Background of the Invention
In general, aerosol spray cans for a variety of aerosol products include a substance to be sprayed, an optional carrier fluid and a propellant. Typically, the propellant is a composition that pressurizes the can and assists in atomization of the substance being sprayed. In the past chlorofluorcarbons (CFC's) were widely used as propellants but, due to environmental concerns, these propellants are now banned by international agreement. The optional “carrier” may also, in some instances, pose an environmental or health hazard. For example, in the case of paints and adhesives, a solvent is included in the composition that is sprayed. The solvent, often a “volatile organic compound” (VOC), ultimately vaporizes, when the paint or adhesive “dries”, to leave behind the sprayed composition. The vaporization of these solvents into the environment has now raised both health and environmental issues: do they pose a risk of adversely affecting human health and air quality? In response, industry has been seeking ways to reduce the amounts of organic solvents present in adhesive and other aerosol sprays that may pose a risk.
A reduction in solvent would also produce other benefits: as the proportion of solvent present in aerosol adhesive decreases, more of the adhesive composition itself is present in the aerosol. This results in reduction of waste to dispose of in the form of the packaging for the adhesive mixture (e.g., empty spray cans).
Thus, for example, spray can-applied adhesive/solvent mixtures containing 20-25% by weight adhesive compound, also known as very high solid content (VHS) adhesives, have become increasingly desirable in the field of contact adhesives because of their use of smaller proportions of organic solvents, and their potential for reduction of health and environmental hazards. Our copending U.S. patent application Ser. No. 09/126,383, entitled “Very High Solids Adhesive” filed Jul. 30, 1998, which is hereby incorporated by reference, discloses such a composition for a VHS adhesive and a method for making the VHS adhesive. In a preferred embodiment, the adhesive is comprised of a resin/rubber/solvent mixture. The resins used typically include polyterpene resins, phenolic resins, phenolic modified terpene resins, aliphatic petroleum hydrocarbon resins, and the like. The rubbers used in the adhesive mixtures generally use a blend of polychloroprene synthetic rubbers. A wide range of solvents may be used depending on the composition of the adhesive with which it must be compatible to form a solution. Thus, the solvents used may include, among others, various chlorinated solvents, ketones, aliphatics, aromatics, alcohols, and esters, or even inorganic solvents such as water.
However, it has been found that in practice using VHS adhesives can be quite difficult. For example, in general, as the solid content of the adhesive increases, the viscosity of the adhesive/solvent mixture increases. When using standard nozzles and buttons on typical aerosol spray cans, the increased viscosity causes the spray pattern of adhesive mixture from the can to be uneven. For example, if a standard can, valve, and button (such as variable valve Model #V8-10-118, with a 906 collar and button Model #166-197-1620-white, both provided by Newman-Green of Addison, Ill.) are used to spray a VHS adhesive/solvent mixture having 30 wt % adhesive, such as neoprene, the spray tends to be uneven. That is, the spray pattern will have varying concentrations across the area of application. It is believed that this generally occurs because the button contains a substantially circular shaped exit port through which the VHS adhesive mixture stream passes so that there is limited or no “fanning” of the spray; the stream exits in a substantially straight line. Additionally, even if some outward “fanning” should occur, the fanning is not controlled and the concentration of the sprayed fluid is not uniform and tends to vary throughout the application area.
Various nozzles for attachment to the spray buttons have been designed to try to overcome the nonuniformity of spray problem. U.S. Pat. No. 4,401,272, issued to Merton et al., on Aug. 30, 1983, and U.S. Pat. No. 4,401,271, issued to Hansen, on Aug. 30, 1983, each disclose nozzles which attach to aerosol spray can buttons. These nozzles do not appear to resolve the issue. For example, the '272 patent discloses that the nozzle is only capable of spraying mixtures with solid content levels up to 11.1%, well below typical VHS levels. When such nozzles are used, the spray tends to be more concentrated at the top and bottom of the spray area and less concentrated near the center of the spray area. The '271 patent provides another attempt at a solution to the “nonuniformity of spray” issue.
As explained above, there is a need for a VHS adhesive/solvent mixture with higher workable solids contents than heretofore known and a device for applying such a mixture substantially uniformly.